The present invention relates to electric arc furnaces and especially to an apparatus for rapid meltdown of raw and scrap metals. More particularly, the invention relates to a vertical shaft furnace employing sidewall-mounted plasma arc torches for rapidly producing a continuous supply of molten metal especially for use in continuous casting machines.
Meltdown of a continuously or semi-continuously supplied metal charge in a furnace for further processing, for example, in a continuous casting machine, requires the input of tremendous amounts of heat energy, some of which is necessarily dissipated or lost to the surroundings by radiation and conduction from the furance. Thus, it is important, not only from economic considerations, but also from an energy conservation standpoint to accomplish meltdown in the shortest possible time so as to improve efficiency and minimize the amount of heat energy lost to the surroundings. It is known that the use of plasma arcs for melting metals in furnaces can significantly improve the meltdown rate because of the much higher temperatures involved, e.g., in some instances, as high as 10,000.degree. to 20,000.degree. K. It is also known to improve the efficiency of heat transfer from a plasma arc torch to a scrap metal charge in a furnace by the selective positioning of plasma arc torches mounted in the sidewalls of the furnace. Such plasma arc furnace apparatus are disclosed, for example, in U.S. Pat. Nos. 3,422,206 and 3,749,803. These prior art furnaces are not, however, readily adapted for continuously supplying molten metal to a continuous casting machine and must be frequently shut down to permit tapping of the molten metal therein with the consequent loss of time and dissipation of substantial amounts of heat energy. Other prior art furnaces employ plasma art torches for cold-mold remelting and refining of consumable metal bars into ingots and are exemplified by the furnace apparatus disclosed in U.S. Pat. Nos. 3,496,280 and 3,736,361.
In general, however, the use of plasma arc torches in metal melting furnaces has been limited to cold-mold remelting furnaces and other types of non-continuously operated furnaces such as those mentioned above. Although vertical fuel-fired shaft furnaces for continuously melting scrap metal are known, for example, from U.S. Pat. Nos. 3,199,977 and 3,788,623, plasma arc torches, so far as known, have not heretofore been utilized in such vertical shaft furnaces to produce a continuous supply of molten metal for casting in continuous casting machines. In conventional fuel-fired furnaces, intermittent or semicontinuous charging of the shaft furnace with a supply of scrap metal materials is readily and advantageously accomplished merely by dropping the material into the furnace via an opening adjacent the upper portion thereof and permitting the material to assume randomly disposed positions and form a vertical column of scrap which descends in the furnace shaft as the charge is melted. After charging, the charge opening is then closed and thereafter intermittently charged as necessary in the same manner with additional scrap material.
As noted in the aforementioned U.S. Pat. No. 3,199,977, this random charging procedure provides a less compact mass of metal charge and thereby enhances the melting rate. Such a time-efficient charging procedure of a vertical shaft melting furnace would not, however, be feasible if plasma arc torches were employed for melting the metal charge since the torch tips normally extend into the furnace beyond the interior wall thereof and in relatively close proximity to the metal charge so as to permit striking and maintaining the arc between the torch electrode and the charge. The torch tips would, therefore, be susceptible to extensive damage by either the dropped scrap material or the descending scrap metal charge.